M-153/U Microphone, Voice Silencer

This is a special microphone designed to keep your voice from being
heard by any one else when speaking. There is a Push To Talk switch
on top of the "nose". The resulting sound is very crisp and easy
to understand. Operationally there is no provision for hearing so
the VOL control on the radio needs to be turned almost to OFF so that the
speaker will not make any noise. To prevent warning tones the ANT
switch should be on the correct letter for the frequency being used (maybe
add some tape to the CHAN switch and the ANT switch. Also be sure
the battery is fully charged to prevent the low battery warning tone.

The manual mentions using the M-153, held to the ear, as a way to hear
the radio output without others overhearing the audio, but I don't see
how that is possible because as far as I can tell the M-153 is wired only
to the A (gnd) and D (Mic) pins of the U-229 connector. The MIC appears
to always be connected to the A & D pins. The PPT switch grounds
the AUDIO "C" pin to A (gnd) when pressed. The AUDIO "B" (ear) pin
is not connected to anything. The only way to use the M-153 as a
silenced earphone would be to have the audio fed to it on AUDIO pin D,
not too practical.

If headphones will be used with the M-153, then a Y
cable will need to be used that allows both to be connected at the
same time.

Side image showing the PTT switch on
the "nose". On the top of the rubber face gasket is the name "Stenomask".
Mouthpiece image showing the microphone
element on the right.